PROJECT SUMMARY Fuchs? endothelial corneal dystrophy is a degenerative disorder of the eye?s corneal endothelium and the most common indication for corneal transplantation. The majority of patients carry a trinucleotide repeat expansion in the TCF4 gene, causing sequestration of the MBNL1 splicing factor and widespread mis-splicing of MBNL1- regulated transcripts. Here we propose to use patient-derived cells to provide a characterization of the molecular pathology of this disease and evaluate the potential of our newly developed RNA-targeting CRISPR/Cas9 system as a therapeutic modality to eliminate toxic repeat expansions in TCF4 RNA transcripts.